This section provides background information related to the present disclosure which is not necessarily prior art. In motor vehicles known from prior art, the waste heat of the engine is used to heat the air intake for the passenger compartment. The waste heat is transported by means of the refrigerant circulating in the engine cooling circuit to the air conditioner, where it is transferred by the heating system heat exchanger to the air flowing into the passenger compartment. The known systems with refrigerant/air heat exchangers, which draw heating power from the cooling circuit of an efficient internal combustion engine of the vehicle propulsion system at low ambient temperatures no longer achieve the required level for a comfortable heating of the passenger compartment to meet the overall heating demand of the passenger compartment. The same holds for systems in vehicles with hybrid propulsion or exclusively electrically operated vehicles.
When the overall heating demand of the passenger compartment cannot be met by the heat from the engine cooling circuit, additional heating measures are necessary, such as electrical resistance heaters (PTC) or fuel-operated heaters. A more efficient option for heating the air for the passenger compartment is a heat pump with air as the heat source, where the refrigerant circuit serves both as a single heating system and an extra heating feature.
Air/air heat pumps of the prior art, which are designed for combined air conditioner and heat pump mode, and consequently also for the heating mode, take up heat from the surrounding air. Power is transferred between the refrigerant and air. Heat pump systems, which cannot dehumidify and heat the air supplied to the passenger compartment at the same time, cannot be operated at low ambient temperatures with surrounding air, that is, with air recirculating from the passenger compartment. Due to the lack of a dehumidifying function, the remaining humidity of the air as well as the water given off in the form of vapor from the passengers would result in fogging of the windows. On the other hand, in regions with temperature climate and ambient temperatures in the range of around 0° to 20° C., it is often demanded of the air conditioning system to operate the evaporator with the goal of dehumidifying the intake air of the passenger compartment.
For traditional air conditioning systems, when ambient temperatures are above 20° C., after reaching thermal comfort, the air being supplied to the passenger compartment is cooled down to around 2° C. to 10° C. and thereby sufficiently dehumidified. But due to the low ambient temperature, the cooled and dehumidified air cannot be supplied directly to the passenger compartment. In vehicles with internal combustion engines, the air is then reheated to the desired intake air temperature making use of the waste heat of the engine. A set point temperature for the passenger compartment of around 20° C. to 25° C. is considered thermal comfort, for example.
In the so-called “reheat” mode, the air being supplied to the passenger compartment is consequently cooled down, and thereby dehumidified, and then slightly heated again. In this operating mode, the required reheating power is less than the cooling power needed to cool and dehumidify the air.
In DE 10 2006 026 359 A1, an air conditioning system is described with a refrigerant circuit for a combined operation in the cooling unit and heat pump mode. The disclosed heat pump system consists of a primary circuit and a secondary passage divided into various sections. The primary circuit contains the components already known from a classical compression refrigerating machine such as a compressor, two heat exchangers, and a throttling element. The secondary passage has an additional heat exchanger operated as a heat pump condenser. The air conditioning system enables a reheating only in the heat pump operation. Furthermore, the heating power given off to the air in the additional heat exchanger of the secondary passage is always greater than the cooling power taken up in the heat exchanger of the primary circuit, designed as an evaporator. Heat pump systems that are configured for this operation in the reheating mode are also known from the prior art.
DE 10 2011 109 055 A1 discloses an air conditioning system for a vehicle with a refrigerant circuit that can operate in the heat pump and the cooling unit mode. The refrigerant circuit here has a compressor, an internal heat exchanger, and an external heat exchanger operating as a condenser in the cooling unit mode. The internal heat exchanger, operated as an evaporator, is coordinated with an internal heating condenser. The air conditioning system furthermore comprises a device for creating an air mass flow that can be thermally coupled to the internal heat exchanger and the internal heating condenser as well as an adjusting device by means of which the flow through the internal heating condenser can be adjusted with a partial air mass flow of the thermally coupled air mass flows. Consequently, the internal heating condenser, receiving a continuous flow as a heat pump condenser is coupled at the air side to the internal heat exchanger configured as an evaporator. The heating power in the reheat mode is regulated by varying the partial air mass flow across the heat pump condenser.
In this case, when operating in the reheat mode the pressure of the refrigerant in the heat pump condenser cannot be adjusted significantly above the pressure level in the external heat exchanger and the intake air for the passenger compartment cannot be heated substantially above the temperature of the ambient air. The pressure in the external heat exchanger is essentially determined by the temperature of the ambient air. With rising temperature of the ambient air, the heating power furnished by the system will rise, but in any case the heating power required for thermal comfort of the passenger compartment will also become less at higher ambient air temperatures. Consequently, at high ambient air temperatures, the system can adequately provide heating power when operating in the reheat mode, but it has a heating power deficit at low ambient air temperatures, for example, at temperatures below 20° C. In order to generate the required heating power, the system must be operated in the pure heat pump mode. Yet in this operating mode, adequate dehumidification of the intake air for the passenger compartment cannot be realized. The refrigerant circuit then switches between the heat pump mode and the cooling unit mode.
In DE 10 2009 028 522 A1 an air conditioning system is disclosed with a refrigerant circuit configured as a heat pump system. The refrigerant circuit likewise has a primary circuit, as a modification of the refrigerant circuit from DE 10 2006 026 359 A1, and a secondary branch with a heat pump condenser. Thanks to a division of the overall refrigerant mass flow into a first partial mass flow through the primary circuit in which the cooling power is transferred to the evaporator and a second partial mass flow through the secondary passage in which the heating power is transferred to the heat pump condenser, an operation in reheat mode is made possible with the heating power regulated independently of the cooling power.
However, in this design of the refrigerant circuit as well, the pressure of the refrigerant in the heat pump condenser cannot be adjusted independently of the ambient temperature and thus cannot be set significantly above the pressure level in the condenser of the primary circuit. Although adequate heating power can be made available when operating in the reheat mode, the temperature level is not sufficient for a comfortable air conditioning of the passenger compartment at ambient air temperatures less than 20° C. To achieve the heating power needed for this, the air conditioning system has to be operated in the pure heat pump mode, but then sufficient dehumidification of the intake air for the passenger compartment cannot be assured. As with the air conditioner from DE 10 2011 109 055 A1, the refrigerant circuit switches between the heat pump mode and the cooling unit mode.